A wireless terminal set, represented by a portable telephone set, is now being lighter in weight, thinner in thickness and smaller in size, and in conformity with the recent tendency, miniaturized parts and a method for mounting them on a substrate are actively developed. A SAW device is a frequency-selecting device utilizing a wave propagated along a surface of an elastic body, and frequently used in a high frequency circuit. The fundamental structure of the SAW device is that interdigital transducers, which reversibly convert electrical signals into surface acoustic waves, are formed on the surface of a piezoelectric substrate. The interdigital transducer is formed by the use of thin film-forming and photolithography technologies. Material of the interdigital transducer is Al in most cases, and its width is in the order of micron to submicron meter.
The SAW device is composed of a piezoelectric substrate and a circuit substrate. The piezoelectric substrate is provided with the interdigital transducers and surface wave propagation areas on its functional surface, and mounted on the mounting surface of the circuit substrate. The functional surface of the piezoelectric substrate and the mounting surface of the circuit substrate are opposed to each other, and the circuit substrate supplies electrical power to the interdigital transducers via electrode pads respectively formed on both the substrates and bumps inserted between the electrode pads.
In order to protect the interdigital transducers with minute widths from the environmental space, it is indispensable to seal the SAW device. Moreover, in order to secure certain propagation of the surface waves, it is necessary to form vibration cavities over the surface wave propagation areas.
Earnest efforts have been made to meet the aforementioned requirements. For example, a SAW device sealed by a metallic casing is the most reliable from the view points of sealing capability, mechanical strength and shielding property against external electro-magnetic waves, but unfavorable from the aspects of cost, weight and size. In the other structure of the SAW device, a hollow is formed on a part of the circuit substrate opposing to the surface wave propagation areas in order to secure the vibration cavity. However, since the shape of the circuit substrate is particular, the cost of production is high, and it is difficult to achieve satisfactory sealing capability.